/* Qilin - a Java Pointer Analysis Framework
 * Copyright (C) 2021-2030 Qilin developers
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation, either version 3.0 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Lesser Public License for more details.
 *
 * You should have received a copy of the GNU General Lesser Public
 * License along with this program.  If not, see
 * <https://www.gnu.org/licenses/lgpl-3.0.en.html>.
 */

package qilin.util.graph;

import java.util.Collection;
import java.util.HashSet;
import java.util.Set;
import java.util.Stack;
import java.util.stream.Collectors;

public interface DirectedGraph<N> {
  Collection<N> allNodes();

  Collection<N> predsOf(final N p);

  Collection<N> succsOf(final N p);

  /* no cache, very slow.*/
  default Collection<N> computeReachableNodes(N source) {
    Set<N> reachableNodes = new HashSet<>();
    Stack<N> stack = new Stack<>();
    stack.push(source);
    while (!stack.isEmpty()) {
      N node = stack.pop();
      if (reachableNodes.add(node)) {
        stack.addAll(succsOf(node));
      }
    }
    return reachableNodes;
  }

  default Collection<N> computeRootNodes() {
    return allNodes().stream()
        .filter(node -> predsOf(node).size() == 0)
        .collect(Collectors.toSet());
  }

  default Collection<N> computeTailNodes() {
    return allNodes().stream()
        .filter(node -> succsOf(node).size() == 0)
        .collect(Collectors.toSet());
  }
}
